Field of the Invention
The present invention relates to capping layers for optical elements, e.g. multilayer mirrors, for use with extreme ultraviolet (EUV) radiation. More particularly, the invention relates to the use of capping layers on optical elements in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask-tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
In the present document, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d (Z) used herein is intended to refer to the direction normal to the substrate or mask surface or parallel to the optical axis of an optical system, rather than implying any particular orientation of the apparatus. Similarly, the term xe2x80x9chorizontalxe2x80x9d refers to a direction parallel to the substrate or mask surface or perpendicular to the optical axis, and thus normal to the xe2x80x9cverticalxe2x80x9d direction.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire a reticle pattern onto the die at ones; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
Optical elements for use in the EUV spectral region, e.g. multilayered thin film reflectors, are especialy sensitive to physical and chemical damage which can significantly reduce their reflectivity and optical quality. Reflectivities at these wavelengths are already low compared to reflectors at longer wavelengths which is a particular problem since a typical EUV lithographic system may have nine mirrors; two in the illumination optics, six in the imaging optics plus the reflecting reticle. It is therefore evident that even a xe2x80x9csmallxe2x80x9d decrease of 1-2% in the peak reflectivity of a single mirror will cause a significant light throughput reduction in the optical system.
A further problem is that some sources of EUV radiation, e.g. plasma based sources, are xe2x80x9cdirtyxe2x80x9d in that they also emit significant quantities of fast ions and other particles which can damage otical elements in the illumination system.
Proposals to reduce these problems have involved maintaining the optical systems at very high vacuum, with particularly stringent requirements on the partial pressures of hydrocarbons which may be adsorbed onto the optical elements and then cracked by the EUV radiation to leave opaque carbon films.
It is an object of the present invention to provide optical elements, including multilayer mirrors, for use in lithographic projection apparatus using extreme ultraviolet radiation (EUV) for the projection beam, that are more resistant to chemical and physical attack.
According to the present invention, this and other objects are achieved in a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; characterised by:
at least one optical element having a surface on which radiation of the same wavelength as the wavelength of said projection beam is incident and a capping layer covering said surface, said capping layer being formed of a relatively inert material.
The optical element may be a beam modifying element such as a reflector, e.g. a multilayer near-normal incidence mirror or a grazing incidence mirror, included in one of the illumination and projection systems: an integrator, such as a scattering plate: the mask itself, especially if a multilayer mask; or any other optical element involved in directing, focussing, shaping, controlling, etc. the projection beam. The optical element may also be a sensor such as an image sensor or a spot sensor;
The relatively inert material in particular should be resistant to oxidation and may be selected from the group comprising: diamond-like carbon (C), boron nitride (BN), boron carbide (B4C), silicon nitride (Si3N4), silicon carbide (SiC), B, Pd, Ru, Rh, Au, MgF2, LiF, C2F4 and TiN and compounds and alloys thereof.
The capping layer should have a sufficient thickness to protect the underlying optical element from attack, so that the capping layer is effecively xe2x80x9cchemically opaquexe2x80x9d, yet not be too thick so as to absorb too much of the incident radiation. To these ends, the capping layer may have a thickness in the range of from 0.5 to 10 nm, preferably from 0.5 to 6 nm and most preferably from 0.5 to 3 nm.
The capping layer may itself have a multi-layer structure, e.g. of two layers, with the outermost layer chosen both for improved chemical resistance and low refractive index at the wavelength of the projection beam to improve reflectivity or transmissivity.
A second aspect of the invention provides a device manufacturing method using a lithographic apparatus comprising
an illumination system for supplying a projection beam of radiation;
a first object table provided with a first object holder for holding a mask;
a second object table provided with a second object holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; said method comprising the steps of:
providing a mask containing a pattern to said first object table;
providing a substrate at least partially covered by a layer of energy-sensitive material to said second object table;
irradiating said mask and imaging irradiated portions of said pattern onto said substrate; characterised in that:
said lithographic projection apparatus comprises at least one optical element having a surface on which radiation of the same wavelength as the wavelength of said projection beam is incident and a capping layer covering said surface, said capping layer being formed of a relatively inert material.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake WEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping) metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0672504.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.